You bring up a great point. For a very long time now FOX has outperformed all the lib press put together in viewership.
Fox regularly boasts of this leadership - its been no secret and they have reveled in their dominance for years.
Anyone not noticing simply didnt want to acknowledge the medias own regularly reported numbers of a conservative media constantly outperforming the combined lib media.
Despite this the left still demeans conservatives who watch FOX.
The term Mainstream seems to presently represent the progressive far left ideologists media favorites--its THEIR mainstream.